MATLAB: Matrix and find

findmatrixmatrix manipulation

Hello actually i have a random matrix done of a lot of 0 and some 1. i need to save the position (i,j) of the first 1 for each row/column. If i have a matrix like this one
     0     1     0     0     0     1     0     0
     0     0     1     0     0     0     0     0
     0     0     1     0     0     0     0     0
doing a(i)=find(matrix(i,:),1) i got a=[2,3,3], and this is ok.
Instead if i have this matrix:
     0     0     0     0     0     0     0     0
     0     0     0     0     0     0     0     0
     0     0     1     0     0     0     0     0
doing the same thing with the find i got error.
Improper assignment with rectangular empty matrix.
why?

Best Answer

You have to decide, what you want as output, if a row does not contain any 1.
n = size(matrix, 1);
a = zeros(1, n);   % Pre-allocate!

for i = 1:n
  found = find(matrix(i,:), 1);
  if isempty(found)
    a(i) = NaN;  % Or Inf, or -1, or 0, or whatever?!
  else
    a(i) = found;
  end
end
[EDITED]: You can define the default value in the pre-allocation to make the code 2 lines shorter:
n = size(matrix, 1);
a = nan(1, n);   % Pre-allocate!
for i = 1:n
  found = find(matrix(i,:), 1);
  if ~isempty(found)
    a(i) = found;
  end
end
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